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1.
We perform axisymmetric hydrodynamical simulations that describe the nonlinear outcome of the viscous overstability in dense planetary rings. These simulations are particularly relevant for Cassini observations of fine-scale structure in Saturn’s A and B-ring, which take the form of periodic microstructure on the 0.1 km scale, and irregular larger-scale variations on 1-10 km. Nonlinear wavetrains dominate all the simulations, and we associate them with the observed periodic microstructure. The waves can undergo small chaotic fluctuations in their phase and amplitude, and may be punctuated by more formidable ‘wave defects’ distributed on longer scales. It is unclear, however, whether the defects are connected to the irregular larger-scale variations observed by Cassini. The long-term behaviour of the simulations is dominated by the imposed boundary conditions, and more generally by the limitations of the local model we use: the shearing box. When periodic boundary conditions are imposed, the system eventually settles on a uniform travelling wave of a predictable wavelength, while reflecting boundaries, and boundaries with buffer zones, maintain a disordered saturated state. The simulations omit self-gravity, though we examine its influence in future work. 相似文献
2.
L. W. Avery 《Solar physics》1976,49(1):141-149
Observations of the continuum microwave flux at 2.8 cm from quiet regions of the solar disc reveal low amplitude, quasiperiodic fluctuations at periods of 234 s and 150 s. For oscillating elements 10 arc seconds in extent, the corresponding peak to peak temperature variations are 230 K and 190 K. The energy flux in the oscillations is estimated to be 2.5x102 ergs cm2 s–1, assuming they are caused by acoustic waves. If the oscillating elements are 1 arc second in extent, the energy flux is comparable to that required for coronal heating.No evidence is found for strong oscillations at periods greater than 250 s, although other authors have claimed microwave detection of strong fluctuations at periods of 280 s and 400 s. 相似文献
3.
An analysis is made of the fine structure in the cosmic ray energy spectrum: new facets of present observations and their interpretation and the next step. It is argued that less than about 10% of the intensity of the helium ‘peak’ at the knee at ≈5 PeV is due to just a few sources (SNR) other than the single source. The apparent concavity in the rigidity spectra of protons and helium nuclei which have maximum curvature at about 200 GV is confirmed by a joint analysis of the PAMELA, CREAM and ATIC experiments. The spectra of heavier nuclei also show remarkable structure in the form of ‘ankles’ at several hundred GeV/nucleon. Possible mechanisms are discussed. The search for ‘pulsar peaks’ has not yet proved successful. 相似文献
4.
S. Larsson 《Astrophysics and Space Science》1987,130(1-2):187-192
The AM Her type system E1405-451 is known to show 1–3 second variability in its optical emission. Our observations show this variability to be due to quasi-periodic oscillations with a coherence time on the order of one minute. The observations also reveal variations in the color of the oscillating light source. These variations are difficult to explain with present cyclotron emission models.Based on observations obtained at the European Southern Observatory, La Silla, Chile.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
5.
Current observational constraints on anisotropies in the CMB (Cosmic Microwave Background) and on the clustering of galaxies have considerably narrowed the range of acceptable models for galaxy formation. Observations of these anisotropies on intermediate and large angular scales provide the best constraint on the amplitude of very long wavelength density fluctuations. We present results from the Tenerife CMB fluctuation experiment (a Jodrell Bank-IAC collaboration) at intermediate angular scales (6 deg) which taken together with recently reported upper limits to fluctuations on small (arc min) angular scales constrain severely the spectral index of initial fluctuations in baryon dominated model universes. Constraints on adiabatic fluctuations in hot and cold dark matter models are also briefly discussed. 相似文献
6.
We present self-consistent, fully coupled two-dimensional (2D) numerical models of thermal evolution and tidal heating to investigate how convection interacts with tidal dissipation under the influence of non-Newtonian grain-size-sensitive creep rheology (plausibly resulting from grain boundary sliding) in Europa’s ice shell. To determine the thermal evolution, we solved the convection equations (using finite-element code ConMan) with the tidal dissipation as a heat source. For a given heterogeneous temperature field at a given time, we determined the tidal dissipation rate throughout the ice shell by solving for the tidal stresses and strains subject to Maxwell viscoelastic rheology (using finite-element code Tekton). In this way, the convection and tidal heating are fully coupled and evolve together. Our simulations show that the tidal dissipation rate can have a strong impact on the onset of thermal convection in Europa’s ice shell under non-Newtonian GSS rheology. By varying the ice grain size (1-10 mm), ice-shell thickness (20-120 km), and tidal-strain amplitude (0-4 × 10−5), we study the interrelationship of convection and conduction regimes in Europa’s ice shell. Under non-Newtonian grain-size-sensitive creep rheology and ice grain size larger than 1 mm, no thermal convection can initiate in Europa’s ice shell (for thicknesses <100 km) without tidal dissipation. However, thermal convection can start in thinner ice shells under the influence of tidal dissipation. The required tidal-strain amplitude for convection to occur decreases as the ice-shell thickness increases. For grain sizes of 1-10 mm, convection can occur in ice shells as thin as 20-40 km with the estimated tidal-strain amplitude of 2 × 10−5 on Europa. 相似文献
7.
Long-slit spectroscopy observations of Uranus by the United Kingdom InfraRed Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s Northern Spring Equinox in December 2007.These spectra were analysed and presented by Irwin et al. (Irwin, P.G.J., Teanby, N.A., Davis, G.R. [2009]. Icarus 203, 287-302), but since publication, a new set of methane absorption data has become available (Karkoschka, E., Tomasko, M. [2010]. Methane absorption coefficients for the jovian planets from laboratory, Huygens, and HST data. Icarus 205, 674-694.), which appears to be more reliable at the cold temperatures and high pressures of Uranus’ deep atmosphere. We have fitted k-coefficients to these new methane absorption data and we find that although the latitudinal variation and inter-annual changes reported by Irwin et al. (2009) stand, the new k-data place the main cloud deck at lower pressures (2-3 bars) than derived previously in the H-band of ∼3-4 bars and ∼3 bars compared with ∼6 bars in the J-band. Indeed, we find that using the new k-data it is possible to reproduce satisfactorily the entire observed centre-of-disc Uranus spectrum from 1 to 1.75 μm with a single cloud at 2-3 bars provided that we make the particles more back-scattering at wavelengths less than 1.2 μm by, for example, increasing the assumed single-scattering albedo from 0.75 (assumed in the J and H-bands) to near 1.0. In addition, we find that using a deep methane mole fraction of 4% in combination with the associated warm ‘F’ temperature profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987-15001), the retrieved cloud deck using the new (Karkoschka and Tomasko, 2010) methane absorption data moves to between 1 and 2 bars.The same methane absorption data and retrieval algorithm were applied to observations of Neptune made during the same programme and we find that we can again fit the entire 1-1.75 μm centre-of-disc spectrum with a single cloud model, providing that we make the stratospheric haze particles (of much greater opacity than for Uranus) conservatively scattering (i.e. ω = 1) and we also make the deeper cloud particles, again at around the 2 bar level more reflective for wavelengths less than 1.2 μm. Hence, apart from the increased opacity of stratospheric hazes in Neptune’s atmosphere, the deeper cloud structure and cloud composition of Uranus and Neptune would appear to be very similar. 相似文献
8.
D. Grassi A. Adriani N.I. Ignatiev F. Colosimo L. Brower A. Coradini 《Planetary and Space Science》2010,58(10):1265-1278
Jupiter’s atmosphere presents limited regions of relatively thin cloud coverage (the so-called ‘hot spots’), which allow thermal radiation by warmer, deeper atmospheric layers to be transmitted directly to space. Hot spots therefore represent a means for probing physical conditions (namely chemical composition) below the main aerosol deck.Forthcoming missions to the Jovian system - Juno and EJSM spacecrafts - will host as payload components spectro-imagers operating in the infrared. Their coverage of 5 μm CH4 transparency windows make them particularly suitable for the investigation of hot spots. This study is an assessment of their retrieval capabilities on the evaluation of gaseous mixing ratios from nighttime observations, on the basis of Bayesian theory.The retrieval performance is evaluated for the JIRAM instrument, a confirmed payload component of Juno. Its data will provide effective constraints on the mixing ratios of water vapor between 40 and 70 km below the reference 1 bar pressure level (between 3.5 and 7 bars). Assuming an a priori correlation length equal to half the scale height, we achieve a minimum retrieval uncertainty of 0.17, once the mixing ratio is given in terms of log10(α), with α being the adimensional mixing ratio (vs. altitude) relative to a given reference profile. The JIRAM-Juno dataset will further allow determination of the ammonia mixing ratio, with a minimum relative retrieval uncertainty of 0.32 in the same altitude range, and of the phosphine mixing ratio, with comparable uncertainty up to the reference altitude.The retrieval performance is evaluated for a second instrument VIRHIS, which is a proposed payload component of Jupiter Ganymede Orbiter (JGO), one of the two spacecrafts of Europa-Jupiter System Mission (EJSM). This instrument has the benefit of higher spectral resolution and extended spectral range, when compared to JIRAM-Juno. Evaluation of the water vapor retrieval shows the uncertainty would be reduced to 0.08 with VIRHIS. The ammonia retrieval range would be expanded up to 10 km (0.66 bar), with a minimum uncertainty value of 0.10.Both instruments will place these measurements in a spatial context due to their simultaneous imaging capabilities, enabling therefore a number of studies covering chemical and dynamical aspects of atmospheric evolution. 相似文献
9.
The radiation chemistry, thermal stability, and vapor pressure of solid-phase carbonic acid (H2CO3) have been studied with mid-infrared spectroscopy. A new procedure for measuring this molecule’s radiation stability has been used to obtain intrinsic IR band strengths and half-lives for radiolytic destruction. We report, for the first time, measurements of carbonic acid’s vapor pressure (0.290-2.33 × 10−11 bar for 240-255 K) and its enthalpy of sublimation (71 ± 9 kJ mol−1). We also report the first observation of a chemical reaction involving solid-phase carbonic acid. Possible applications of these findings are discussed, with an emphasis on the outer Solar System icy surfaces. 相似文献
10.
TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution. 相似文献
11.
Y. D. Zhugzhda 《Solar physics》1972,25(2):329-338
Summary Tunneling of surface waves (which are also called non-propagating or evanescent mode) in isothermal atmosphere is considered. Tunneling of 5-min oscillations in solar atmosphere is discussed. Phase lead of chromospheric oscillations with respect to photospheric oscillations (Tanenbaum et al., 1971) can be explained by tunneling only. 相似文献
12.
Erich Karkoschka 《Icarus》2011,215(1):439-448
The interior rotation and motions in giant planets have generally been probed only at radio wavelengths from spacecraft near the planet, except for Jupiter’s radio emission detectable from Earth. Here I suggest that Neptune’s interior can be indirectly probed at visible wavelength by tracking 10 features that are connected with a stationary latitudinal speed pattern of 7 m/s amplitude. All 10 features remained aligned at the same longitude throughout the Voyager observation period in 1989. Two of them, the South Polar Wave and South Polar Feature, have been observed from Earth for ∼20 years, but their extraordinary rotational stability was never recognized. They probably pinpoint Neptune’s rotational period (15.9663 ± 0.0002 h), one of the largest improvements in 346 years of measuring the giant planets’ rotations. The previous best estimate of Neptune’s rotational period (16.108 ± 0.006 h) was based on Voyager 2 radio data (Lecacheux, A., Zarka, P., Desch, M.D., Evans, D.R. [1993]. Geophys. Res. Lett. 20, 2711-2714). The new result suggests an upward revision of the mass of Neptune’s core. This finding may also question the accepted value of Uranus’ rotational period. The first reliable wind measurements within 15° of Neptune’s South Pole, based on tracking four features in Voyager images, show a 300 m/s eastward jet peaking near 76° South, while the area within 4° of the South Pole seems to be rotationally locked to the interior. These new observations of the stationary features and winds could address the long-standing question about the depth of the atmospheric circulation and may allow some constraints on convection currents in Neptune’s interior. 相似文献
13.
We use a radiative-conductive-convective model to assess the height of Pluto’s troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto’s atmosphere are negligible. The surface pressure is found to be slightly higher ( μbar in 1988, μbar in 2002, and 18.5 ± 4.7 μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be . Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto’s lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results. 相似文献
14.
Leigh N. Fletcher G.S. Orton A.A. Simon-Miller M.H. Wong P.G.J. Irwin P.A. Yanamandra-Fisher 《Icarus》2011,213(2):564-580
Mid-infrared 7-20 μm imaging of Jupiter from ESO’s Very Large Telescope (VLT/VISIR) demonstrate that the increased albedo of Jupiter’s South Equatorial Belt (SEB) during the ‘fade’ (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB (7-17°S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool band of high aerosol opacity (the SEB zone at 11-15°S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of 1.0 ± 0.5 K), as well as the increased aerosol opacity at 4.8 and 8.6 μm, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free ‘brown barges’ (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. Upper tropospheric temperatures (150-300 mbar) remained largely unchanged during the fade, but the cool SEBZ formation was detected at deeper levels (p > 300 mbar) within the convectively-unstable region of the troposphere. The SEBZ formation caused the meridional temperature gradient of the SEB to decrease between 2008 and 2010, reducing the vertical thermal windshear on the zonal jets bounding the SEB. The southern SEB had fully faded by July 2010 and was characterised by short-wave undulations at 19-20°S. The northern SEB persisted as a narrow grey lane of cloud-free conditions throughout the fade process.The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. These changes occurred within cloud decks in the convective troposphere, and not in the radiatively-controlled upper troposphere. NH3 ice coatings on aerosols at p < 800 mbar are plausible sources of the suppressed 4.8 and 8.6-μm emission, although differences in the spatial distribution of opacity at these two wavelengths suggest that enhanced attenuation by a deeper cloud (p > 800 mbar) also occurred during the fade. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air. 相似文献
15.
The structure of the periodic solutions of the Störmer problem, representing the magnetic field of the Earth, is examined by considering the equatorial oscillations of the charged particle and their vertical bifurcations with meridian periodic oscillations. An infinity of new families of simple-periodic oscillations are found to exist in the vicinity of the thalweg and four such new families are actually established by numerical integration. 相似文献
16.
We present results from coronagraphic imaging of Mercury’s sodium tail over a 7° field of view. Several sets of observations made at the McDonald Observatory since May 2007 show a tail of neutral sodium atoms stretching more than 1000 Mercury radii (Rm) in length, or a full degree of sky. However, no tail was observed extending beyond 120 Rm during the January 2008 MESSENGER fly-by period, or during a similar orbital phase of Mercury in July 2008. Large changes in Mercury’s heliocentric radial velocity cause Doppler shifts about the Fraunhofer absorption features; the resultant change in solar flux and radiation pressure is the primary cause of the observed variation in tail brightness. Smaller fluctuations in brightness may exist due to changing source rates at the surface, but we have no explicit evidence for such changes in this data set. The effects of radiation pressure on Mercury’s escaping atmosphere are investigated using seven observations spanning different orbital phases. Total escape rates of atmospheric sodium are estimated to be between 5 and 13 × 1023 atoms/s and show a correlation to radiation pressure. Candidate sources of Mercury’s sodium exosphere include desorption by UV sunlight, thermal desorption, solar wind channeled along Mercury’s magnetic field lines, and micro-meteor impacts. Wide-angle observations of the full extent of Mercury’s sodium tail offer opportunities to enhance our understanding of the time histories of these source rates. 相似文献
17.
A recently developed treatment of partition functions in the equation of state (Mihalas, Hummer, Däppen, MH&D) has led to a substantial improvement in the agreement between observed and theoretically predicted solar p-mode oscillation frequencies. The MH&D equation of state is a realization of the free-energyminimization method, based on the so-called chemical picture, in which ionization and dissociation reactions are assumed to be those that maximize entropy, or equivalently, minimize the free energy. An alternative equation of state has recently been developed at Livermore. It realizes a virial expansion of pressure, and is based on the physical picture, in which explicitly only fundamental species (i.e., electrons and nuclei) appear. Results of a first comparison between thermodynamic quantities of the MH&D and Livermore equations of state are presented. For simplicity, a mixture with only hydrogen and helium (90% H and 10% He by number) is chosen. The comparison is made for a low-density and a high-density case. In the first case, the conditions are those of the hydrogen and helium ionization zones of the Sun, in the second case those of the solar centre. In both cases, the MH&D and Livermore results agree strikingly, despite the very different formalisms they are based on. 相似文献
18.
Periodic Doppler width fluctuations have been observed in Fe xiv spectra above an active region. The oscillations have a period of 6.1 ± 0.6 min and a peak-to-peak amplitude of 0.07 ± 0.006 Å. The amplitude of the oscillation increases with height above the limb, and is enhanced at specific heights where we marginally detect line center intensity oscillations. The intensity fluctuations have a period of 6.1 min, an amplitude of 2.0 ± 1.4%, and are 180 ° out of phase with the width oscillations. A comparison region in the quiet corona showed no evidence of oscillatory phenomena.Visiting Student, Sacramento Peak Observatory.NAS/NRC Resident Research Associate.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 相似文献
19.
Leigh N. Fletcher Kevin H. Baines Adam P. Showman Glenn S. Orton C. Merlet 《Icarus》2011,214(2):510-533
The latitudinal variation of Saturn’s tropospheric composition (NH3, PH3 and AsH3) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH3 spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH3 dominates the morphology of the VIMS spectrum, and high-altitude PH3 at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH3 is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH3 shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH3 and upper-tropospheric PH3 and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH3, AsH3 and the lower cloud deck). 相似文献
20.
Keck near-infrared images of Neptune from UT 26 July 2007 show that the cloud feature typically observed within a few degrees of Neptune’s south pole had split into a pair of bright spots. A careful determination of disk center places the cloud centers at −89.07 ± 0.06° and −87.84 ± 0.06° planetocentric latitude. If modeled as optically thick, perfectly reflecting layers, we find the pair of features to be constrained to the troposphere, at pressures greater than 0.4 bar. By UT 28 July 2007, images with comparable resolution reveal only a single feature near the south pole. The changing morphology of these circumpolar clouds suggests they may form in a region of strong convection surrounding a neptunian south polar vortex. 相似文献